I don't know about year long Christmas, but a Club for public holidays could work!
As for Christmas Pokemon, it could just be Pikachu in a Christmas hat! I'm pretty lucky that the character in my picture has an 'official' Christmas outfit which inspired a bunch of fanart, so yours could be as simple as a Pokemon wearing Christmas stuff as I'm sure fanart of lots of Pokemon like that exists.
I know how you feel about Challenges! Even now, I find it a bit...intimidating? (probably not the right word) to post in other sub-forums.